The properties of nanomaterials are different from their bulk counterparts when the characteristic dimension is reduced to nano and atomic level. For example, In Anomalous production of vacancy clusters and the possibility of plastic deformation of crystalline metals without dislocations Kiritani et al. found that the deformation of face-centered-cubic structure bulk materials is realized mainly through dislocation glide, while the deformation of face-centered-cubic structure thin film in nano scale is closely related to the substantial amount of vacancies inside the material; In Discrete plasticity in sub-10-nm-sized gold crystals Zheng et al. found that the mechanism dominating plastic deformation is the nucleation of partial dislocation in single crystal gold with dimensions less than 10 nm, different from the mechanism of plastic deformation dominated by the Frank-Read source in gold bulk materials; In Strong crystal size effect on deformation twinning Yu et al. found that for titanium alloy single crystal the stress required for deformation twinning increases drastically with decreasing sample size. It is of great theoretical and practical importance to study the deformation mechanism of materials and its influence on the mechanical properties for further development of materials with high performance.
The microstructure of materials in sub angstrom, nanometer and atomic scale can be studied by TEM. The latest spherical aberration correction transmission electron microscope can study the microstructure of materials with a spatial resolution of less than 0.1 nm. Apart from the static characterization of microstructures, researchers and manufacturers around the world have developed different kinds of TEM in situ characteristic techniques to study the correlation between the microstructure of materials and their mechanical, thermal, electrical, optical, and various kinds of coupling properties. These developments have greatly enriched the characterization ways in revealing all kinds of physical mechanism of materials. Among these in-situ deformation techniques, the in-situ mechanical measurement of nanomaterials has been widely payed attention to, because that the stability and service life of the materials are mainly determined by their mechanical properties. Using the in-situ mechanical testing, it is easy to study the microstructural evolution of materials under the condition of external force which can guide the understanding of the elastic/plastic deformation mechanism of materials.
At present, the loads applied in in-situ mechanical experiment in TEM are usually tensile load and compressive load. Compared to tensile load, compressive load is more applicable in the study on brittle materials such as ceramics, since brittle materials will break after the yield point under tensile load, which makes it impossible to in situ observe the process of plastic deformation and in turn hinters the precise analysis of materials' deformation mechanism.
In-situ nanoindentation/compression experiment in TEM has obvious advantages compared to ex-situ TEM experiment in which observation and analysis is carried out after unloading. In the ex-situ experiment, the evolution of microstructure can not be observed when the indenter is pressed in, thus it is hard to deduce the correlation between the microstructure and mechanical properties of materials by the fragmented microstructure. The evolution of microstructure during the overall process of indenter pressed—stay—withdrawal can be observed and analyzed in real time by in-situ nanoindentation, thus provides high chance to study the relationship of microstructure-mechanical properties in sub angstrom, atomic and nano scale.
In the present studies, in-situ nano compression/indentation experiments were mainly carried out by commercial in-situ nano compression/indentation TEM holder. In the paper An in-situ nanoindentation sample holder for a high-voltage TEM (year 1998), Wall et al. designed the first generation of in-situ nanoindentation sample holder applicable to high-voltage TEM, in which detachable sample loading stage and piezoelectric driving indenter were used. The indenter was first adjusted to a certain range from the sample by a coarse positioning device of gear motor, and then was further driven to get close to and pressed into the sample by a piezoelectric ceramics driver. In the paper development of a nanoindenter for in-situ TEM (year 2001), Stach et al. applied the sample holder to low-voltage electron microscope, and at the same time improved the displacement resolution of the indenter, making the press-in process more stable and controllable. Svensson et al. eliminated the vibration generated during the coarse positioning by using displacement accumulation of piezoelectric ceramics. In the paper A miniaturized TEM nanoindenter for studying material deformation in-situ (year 2006), Bobji et al. further improved the control of press-in track by coupling a signal sensor with controllable flexure hinge, by which the force-displacement curve in the press-in process could be obtained. The indenter in the above in-situ nanoindentation experiments was in tapered shape with curvature radius of tens to hundreds of nanometer. The thickness of the TEM sample was also tens to hundreds of nanometer. Therefore, it is hard to exactly control the contact of the indenter and the sample, thus it is time consuming to conduct the indentation and compression tests in TEM. In the paper Advanced TEM triboprobe with automated closed-loop nanopositioning (year 2010), Lockwood et al. developed an image feedback system to precisely positioning the indenter and the specimen, thus greatly improved the efficiency of in-situ experiment.
Some of the compression/indentation TEM holders have been commercialized by Hysitron, Nanofactory companies and etc. and have been successfully applied to the study of the mechanical properties structure relationship of materials under indentation and compression. Until now, all the commercial holders are driven by piezoelectric ceramics, which can only be placed in the middle or front of sample holder because of its relatively large volume, which disables the Y-axis tilt of the holder and limits the obtaining of the evolution of microstructure at atomic and sub angstrom scale. In the patent A double-axis tilt in-situ nanoindentation device for TEM (patent application number: 201320574347.8), Han et al. designed a platform by which in-situ nanoindentation under the condition of double-axis tilt can be realized in TEM. The design is simple and low-cost, but it is difficult to precisely positioning the indenter as close and parallel as possible to the specimen.
This invention designs a platform for in-situ nanoindentation experiment under the condition of double-axis tilt for TEM. This technique is simple and applicable for mass production. With this technique, we can conduct nanoindentation, compression and bending of the sample and simultaneously obtain the evolution of microstructure at nano, atomic and sub-angstrom scale, which provides an advantageous tool to reveal the deformation mechanism of the materials.